Vitreous enamel resistor

ABSTRACT

Fused vitreous enamel wire-wound resistor formed by roll coating the surface of the wire and core with a fluid vehicle containing a fused ground vitreous enamel frit dispersed therein, and then heat treating so as to drive off the vehicle and remelt the fused enamel frit.

United States Patent Helgeland et a1.

VITREOUS ENAMEL RESISTOR inventors: Walter Helgeland, Nashua; James B. Lazott, Bedford; Philip .1. Duke, New Boston; William J. Taggart, Nashua, all of N.1-1.; George W. Heacox, Pepperell, Mass; Abe Brandsma, Nashua, N11.

Assignee: Sprague Electric Company, North Adams,

Mass.

Filed: Dec. 18, 1969 Appl. No.: 886,240

US. Cl. ..l17/2l5, 117/70 A, 117/70 B, 117/l05.2,117/111R,117/125,117/229, 338/265,

Int. Cl. ..B44d 1/14, B44d 1/02 Field of Search ..117/70 B, 70 C, 215, 227, 229,

117/111 R, 111 F, 105.2, 70 A, 125; 118/233, 110, 244; 29/613; 338/302, 264, 265, 269

[ Feb.29,1972

[56] References Cited UNITED STATES PATENTS 2,844,693 7/1958 Rigterink ..l17/70 B X 2,708,172 5/1955 Robson et 21.... ....1 17/70 B 1,691,297 11/1928 Miller ..118/233 2,394,591 2/1946 Bricker ..l17/215 X 2,712,187 11/1956 Katzetal. ..117/11l RX Primary Examiner-Alfred L. Leavitt Assistant Examiner-C. K. Weifi'enbach Attorney-Connolly and Hutz, Vincent H. Sweeney, James Paul OSullivan and David R. Thornton 1 1 ABSTRACT Fused vitreous enamel wire-wound resistor formed by roll coating the surface of the wire and core with a fluid vehicle containing a fused ground vitreous enamel frit dispersed therein, and then heat treating so as to drive off the vehicle and remelt the fused enamel frit.

2 Claims, 3 Drawing Figures PATENTEDFEB29 I972 3, 545 7 4 mgy W/ I lu VITREOUS ENAMEL RESISTOR BACKGROUND OF THE INVENTION The present invention relates to vitreous enamel resistors, and more particularly to wire-wound resistors in which the wire winding is embedded in an in situ fused vitreous enamel protective layer. I

Prior art method for forming an enamel layer involve the steps of first forming a clay slip. The clay slip may be prepared in any suitable manner. For example the clay, other solids and the water may be mixed and ball-milled to obtain the desired dispersion of all the solids in the water. The resistor is next coated by dipping it in the slip or by allowing the slip to flow upon the resistor thereby converting the resistor. The covered resistor is then dried in a normal atmosphere and fired in a kiln heated to approximately 700 C. in order to fuse the enamel. This process is normally repeated three times so as to build up the enamel coating to a suitable thickness. The leads of the resistor normally become coated during this process thereby necessitating an additional cleaning operation whereby the enamel coating on the leads is sandblasted off. The heat from three separate firings tends to oxidize the resistor leads and stress the resistance winding. Also the high heat of the firing tends to expand the resistance winding thereby creating a greater potential for shorting.

Therefore it is an object of this invention to provide a vitreous enamel resistor manufactured by a simplified process requiring fewer enamel coatings together with a simplified cleaning operation.

It is a further object of this invention to provide an improved vitreous enamel resistor capable of withstanding increased stresses due to the different coefficients of expansion of the resistive elements and coating.

SUMMARY or THE INVENTION A wire-wound resistor embedded in an in situ fused vitreous enamel protective layer is provided in accordance with this inresistive winding or to produce any potential risk of the.

windings shorting. The roll-coating process keeps the resistor leads free from enamel thereby eliminating the extra leadcleaning operation required by the standard kiln-fired process. Also shielding of the leads during heat treatment prevents their oxidation and allows them to be pretinned before coating. The enamel coating comprises a cellular structure which helps take up the stresses caused by dissimilar thermal coefficients of expansion.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a cutaway view of a wire-wound vitreous enamel resistor formed in accordance with this invention;

FIG. 2 shows a number of wire-wound resistors being roll coated with a fluid vehicle containing a ground fused vitreous enamel frit dispersed within it; and

FIG. 3 shows a number of wire-wound resistors being heat treated by absorbing radiant energy while passing between quartz lamps.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a wire-wound resistor formed in accordance with this invention. The resistor core is generally composed of a ceramic material for example, porcelain, magnesium silicate or magnesium aluminum silicate. The resistor core 10 is wound in a conventional manner with electrical resistance wire 12, which normally would be composed of an iron-chromium-aluminum alloy. Alternatively however nickel-chromium alloy or nickel-copper alloy wires may be utilized. The wire 12 is securely held in place by means of nickeled steel end caps 14 which are thermally shrunk or swaged in place to securely hold them to the ends of ceramic core 10 and to which the ends of the wire 12 are welded to form, an electrically conducting unit from one end cap 14 through the wire 12 to the other end cap 14. Terminal leads [6 are connected to each cap 14 as by welding, brazing or soldering with high. temperature solder. Vitreous enamel coating 18 comprises two layers of vitreous enamel which are applied in accordance with the method of the invention and which have an inherent cellular structure thereby taking up stresses The aforementioned resistor is enameled by first roll coating the wire-wound core as shown in FIG. 2 with a frit of fine I ground fused vitreous enamel dispersed in a suitable vehicle. The frit is produced by starting with suitable raw materials for enamel which are mixed and then smelted at a temperature of approximately 700 C. until reaction is complete. The molten enamel is immediately poured into cold water with agitation to quench the glass and break it into small pieces producing the so-called frit. This fritted glass is then ground in a ball mill through a ISO-mesh screen, and then the ISO-mesh enamel frit is dispersed in a suitable vehicle such as water. The vehicle should contain suitable binders such as ethyl cellulose and solvents in order to control the drying rate of the vehicle. Also the vehicle may contain such additives as alumina or finely divided silica to control viscosity and flow.

FIG. 2 illustrates an apparatus for roll coating the frit which can be adapted to standard mass production techniques. The sides of tray 21 are notched so as to support a number of wire wound resistors by their terminal leads leaving them free to rotate on their central axis. Tray 21 is moved in a horizontal direction from left to right over rollers 22 and 23 by conventional means such as belts and pulleys. Tray 21 is positioned so that the resistors supported by it come into contact with the top surfaces of rollers 23 and 22. The bottom of roller 23 is immersed in the vehicle containing the frit of fine ground fused vitreous enamel dispersed within it which is shown in FIG. 2 as being confined'within the open-ended container 25. A uniform coating of enamel frit adheres to the surface of roller 23 as it is rotated in a counterclockwise direction by conventional means. The top of roller 23 contacts the surface of the wire-wound cores rotating them in a clockwise direction and coating them with a uniform layer of enamel frit. The coated resistors subsequently pass over roller 22 which is also rotated in a counterclockwise direction and which removes any excess coating from the resistors. The obvious advantage of this type of application procedure over the prior art dipping process is that the leads remain clean and are not coated with enamel thereby eliminating the subsequent sandblasting of the leads. Also a more uniform coating is achieved by roll coating than is possible by the standard kiln process of allowing the slip to flow upon the resistor. It is sufficient to apply two coats by the roll coating procedure whereas three coats are required by the dipping process.

The firing operation by this procedure is less critical than by the standard kiln process because all that is required by this process is a remelting of the enamel frit which has been previously fused together with sufiicient heat, whereas by the standard kiln process there must be a high enough heat to fuse the clay coating.

FIG. 3 illustrates an apparatus by which roll-coated resistors may be fired. Housing 31 which may be composed of a reflective metal such as brass contains four quartz lamps 33 for heating elements. The quartz lamps traverse the length of the housing and generate temperatures in the range of l,000 C. The coated resistors supported by tray 21 move through housing 31 in a horizontal direction from left to right by conventional means and absorb heat by radiation. The movement of the resistors through housing 31 is timed so that the coated resisters reach a temperature of 700 C. just before they emerge.

from the housing. There is very little oxidation of the leads as they pass through the housing, due to the fact that the coated resistors only reach a temperature of 700 C. as they emerge from the housing. Also the terminal leads of the resistors emerge from the side of the housing and are shielded by the housing thereby further reducing the oxidation of the leads.

The rolled vitreous enamel coating has a cellular structure which helps take up the stresses due to mismatches in temperature coefficients. Also resistors coated in accordance with this invention have only two enamel layers as opposed to the three layers required by the standard kiln firing thereby simplifying the manufacturing process. Another feature of the present invention is that the reduced heat absorbed by the resistor during temperature treatment together with the shielding of the terminal leads enables the fusion to be carried out on cores that have ordinary tinned copper terminal leads.

The resistors of the present invention may be wound with a nickel-chromium alloy wire which initially has a near zero temperature coefficient of resistivity. The high temperatures and long heating times of the standard kiln process are known to affect the temperature coefficient of resistivity by what is cient of resistivity is noticeably less atfected by the method of shorting during firing.

The roll-coating process of this invention may be used in combination with the flame-spraying process wherein fused enamel is milled to a fine frit and then flame sprayed over the surface of the resistor. The undercoat would be applied by the roll-coating method of this invention with all the advantages thereto. The overcoat would be applied by flame spraying which forms a very dense outer covering thereby making the resistor impervious to moisture, and greatly increasing the dielectric withstanding voltage of the enamel coating. The outer coating of flame-sprayed vitreous enamel is completed so rapidly that the resistor assembly is subjected to the necessary high temperatures for not more than about 10 seconds.

What is claimed isf l. A method of forming a wire-wound resistor coated with an in situ fused vitreous enamel protective layer, the method comprising the steps of: breaking and grinding a fused vitreous enamel into a fine frit; dispersing said vitreous enamel frit into a fluid vehicle so as to fonn a slip; roll coating said slip onto the surface of a nickel-chromium alloy wire-wound resistor having pretinned copper terminal lead without coating said leads; shielding said leads; and heat treating said coated resistor to a temperature not substantially higher than 700 C. to form a fused vitreous enamel protective layer.

2. The method of claim 1 includingafurther step of applying an outer coating of vitreous enamel over said protective layer by flame spraying a vitreous enamel frit onto said layer. 

2. The method of claim 1 including a further step of applying an outer coating of vitreous enamel over said protective layer by flame spraying a vitreous enamel frit onto said layer. 